The imperative for sustainable water management
The imperative to make energy and resource consumption more sustainable is prompting a critical reconsideration of all human endeavors, including urban water management. Water services consume a substantial amount of energy, and wastewater contains valuable resources, including water, heat, organic matter, and essential plant nutrients. To make urban water systems more sustainable, a paradigm shift is needed.
One of the proposed strategies is source separation coupled with anaerobic co-digestion, which appears to be an effective means of recovering energy, water, and nutrients. However, existing centralized infrastructure that serves tens to hundreds of thousands of people is difficult to alter, and the technologies needed to realize this strategy are challenging to implement in single-family homes.
This article explores the potential of decentralized water systems at the scale of a city block, using a quantitative model to assess the technical and economic feasibility of a representative system, as well as its environmental impacts. We complement the decentralized water system with vertical farming, photovoltaic energy generation, and rainwater harvesting to realize potential synergies associated with on-site use of the recovered resources.
Harnessing the benefits of source separation
Separation of wastewater into three different components – yellow water (urine), brown water (feces and flush water), and grey water (everything else) – has the potential to reduce the costs of recovering water, energy, and nutrients compared to the conventional approach of treating the combined wastewater streams.
Black water (combined brown and yellow water), which constitutes roughly 20% of the volume of household wastewater, contains about 90% of the carbon and nitrogen and 80% of the phosphorus discharged by households. Treating this resource-rich stream separately, such as through anaerobic digestion, is a proven means of recovering energy and water.
Urine-derived fertilizers, such as P precipitates (Ca3(PO4)2(s)) and stabilized urine liquid concentrate, exhibit higher purity than conventional biosolids. The production of fertilizers in proximity to their point of use also reduces costs associated with transportation and integration into a larger supply chain.
Integrating vertical farming for enhanced sustainability
To address the potential for using recovered nutrients for the cultivation of high-value crops, we considered commercially available modular, hydroponic systems capable of growing tomatoes, lettuce, strawberries, spinach, and mushrooms. The allocation of production area for each crop was determined with an optimization model that factors in capital and operational costs, marketable weight per plant, crop harvest cycle, consumer pricing, and per capita consumption.
The nutrients recovered by the decentralized treatment system would not exactly match the amount needed to grow the produce. Using the optimal mix of crops, the recovered nutrients could supply all the necessary phosphorus, magnesium, potassium, and sulfur for the vertical farm, as well as 56% nitrogen and 31% calcium. To ensure optimal growth, nitrogen, calcium, and other trace nutrients must be added to the system.
The theoretical daily (organic and locally grown) produce from the vertical farming operation would exceed the United States national average consumption for residents of the housing block. Assuming current market prices, the value of the produce would offset the vertical farming investment and operational costs in approximately 10-12 years.
Harnessing solar energy for self-sufficiency
The decentralized system’s appeal further increases when considering energy from photovoltaic systems, especially in locations with high energy costs for water import and distribution. Representing diverse Köppen-Geiger climates, the cities of Barcelona, Toronto, Santiago de Chile, Hong Kong, and Miami were selected for analysis.
The installations are anticipated to offset an average of 12 ± 3% of the estimated total domestic electricity demand. The decentralized system and the vertical farm represent approximately 2.0 ± 0.7% and 2.4 ± 0.2% of the total annual energy demand of the considered development, respectively. Consequently, the integrated photovoltaic installation would reduce grid energy consumption by approximately 8-11% compared to a development lacking these features.
Economic and environmental advantages of decentralized systems
A detailed cost and energy analysis indicates that the performance of the decentralized system is similar to that of the centralized system. However, when factoring in cost offsets from food production, rainwater harvesting, and energy generation, the decentralized system becomes substantially more cost-effective, potentially reducing costs by half or more compared to the centralized system.
Considering energy produced through photovoltaics and food waste digestion, the decentralized system demonstrates a substantial decrease in grid energy consumption, potentially using half or less than the centralized system. The anaerobic digester’s ability to recover energy and heat from brown water and food waste gives decentralized systems the potential to offset over 50% of their total energy demand.
The decentralized system’s appeal is further enhanced by the potential for indirect payback mechanisms, such as the availability and sale of locally grown, water-efficient, and organic produce, as well as the energy cost savings from the integrated photovoltaic system. These payback mechanisms could deliver a return on investment in about 10-15 years, avoiding the costs associated with the expansion of existing potable water supply.
Overcoming barriers to adoption
While many innovative decentralized urban water solutions have been around for years, real implementation at scale remains challenging. Arguably, the most significant barrier is the lack of broad institutional support, which is primarily rooted in technological inertia reinforced by the institutions responsible for urban water management.
Successful transitions require the adaptation of institutional arrangements and new regulatory frameworks. Collaborative policies at local and national levels, driven by the efforts of utility and government leaders, will be crucial for the creation of a supportive regulatory environment. Well-designed subsidies can also relieve financial pressure and incentivize adoption, especially in the early stage of the transition.
Engaging stakeholders (regulators, utilities, developers, and the public) is crucial to addressing potential barriers, building legitimacy for unfamiliar technologies, reforming institutions to accommodate hybrid water systems, and creating targeted incentives. Transparent management and regulation of these projects, involving trusted independent oversight groups, can provide an additional layer of public reassurance and scrutiny.
Conclusion
Decentralized water systems can serve as a cornerstone of efforts to enhance resource efficiency and improve the resilience of cities. By efficiently recovering water, energy, and nutrients, and integrating synergistic strategies like vertical farming and solar energy generation, these systems offer a compelling case for breaking the lock-in effect in urban water management.
While overcoming the institutional and regulatory barriers to widespread adoption remains a challenge, the potential benefits of decentralized systems in terms of cost-effectiveness, environmental sustainability, and community resilience make them a promising solution for the urban water challenges of the future. By aligning with broader societal goals and fostering public-private partnerships, decentralized water systems can pave the way for a more sustainable and resilient urban future.
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